The present invention relates to a photographic lens barrel disposed in a camera or similar device, and in particular to a lens barrel which drives plural lens units in the optical axis direction using a multiple thread screw and cam action.
A prior art example of a zoom lens barrel of a camera is disclosed in Japanese Laid-Open Patent Publication JP-A-8-5887. This prior art lens barrel includes a rotary advance movement tube. Due to the action of a multiple thread screw and a cam tube inserted into an internal circumference of the rotary advance movement tube, rotary advance movement tube rotates while moving in an optical axis direction. This combined multiple thread screw and cam action causes a first, second, and third lens group to move in the optical axis direction, enabling the lens barrel to perform a zooming operation. In addition, the lens groups also move as a result of still further cam actions. As a result of this zooming operation, the lens barrel extends from a collapsed position with respect to a camera body, to a wide angle end position, and to a telephoto position.
The total amount of movement of each lens group during a zooming operation is dependent upon three different types of movement. First, the amount of movement of the lens groups is dependent upon the amount of movement of the rotation advance tube that results from the described multiple thread screw action. Second, the amount of movement of the lens groups is dependent upon the amount of movement of the cam tube with respect to the rotation advance tube that results from the cam action accompanying the rotation of the rotation advance tube. Third, the amount of movement of the lens groups is dependent upon the amount of movement of each lens group with respect to the cam tube that results from the other cam action.
In a prior art zoom lens barrel, the cam tube rotates to move the lens barrel from a collapsed position to a wide angle end position, extending the lens barrel in an optical axis direction. In this wide angle end position, a third lens group closest to the film surface that corresponds to a first lens unit is not extended. On the other hand, a first and a second lens group that correspond to a second lens unit extend a comparatively large amount in the optical axis direction. As the lens barrel is moved from the wide angle end position to a telephoto end position, the third lens group must be moved a comparative distance along the optical axis. More particularly, when moving from the collapsed position to the wide angle end position, the extension of the third lens group that results from the extension of the rotation advance tube retracts the third lens group toward the film surface with respect to the cam tube. As a result, the third lens group is extended a relatively short distance for the wide angle end position. But, when driving from the wide angle end position to the telephoto end position, it is necessary for the third lens group to extend a relatively large amount with respect to the cam tube.
In order to properly extend the prior art lens barrel, it is necessary that a cam groove of the cam tube include regions which cause the third lens group to move in a retraction direction. These regions increase the necessary length of the cam tube in the optical axis direction. As a result of this increased length of the cam tube, the thickness of the camera increases, making the camera larger. On the other hand, when the amount of retraction of the third lens group is suppressed by not including a retraction direction driving region in the cam groove, the amount of movement of the rotation advance tube in the optical axis direction is also reduced. In order to reduce the amount of movement of the rotation advance tube in the optical axis direction, the amount of rotation must also be reduced. In doing so, since the amount of movement of the first and second lens groups is relatively large, not only is it necessary to make the cam tube long, but the inclination of the cam groove for movement of the first and second lens groups must also be steep. This steep inclination of the cam groove that is necessary for the movement of the first and second lens groups causes the load of the first and second lens groups to increase. As a result of this increased load, accuracy of movement of the first and second lens groups is corrupted.